Plasma and Fusion Research: Rapid Communications

Volume 12, 1202020 (2017)

Current Start-Up Using the New CHI System
Kengoh KURODA, Roger RAMAN1) , Kazuaki HANADA, Makoto HASEGAWA, Takumi ONCHI,Masayuki ONO2) , Thomas JABOE1) , Brian A. NELSON1) , Masayoshi NAGATA3) ,Osamu MITARAI4) , Kazuo NAKAMURA, Hiroshi IDEI, John ROGERS1) , Shoji KAWASAKI,Takahiro NAGATA, Arseniy KUZMIN, Shinichiro KOJIMA, Osamu WATANABE,Aki HIGASHIJIMA, Yuichi TAKASE5) and Atsushi FUKUYAMA6)
Kyushu University, Kasuga, Fukuoka 816-8580, Japan1)
University of Washington, Seattle, WA 98195, USA
2)
Princeton University, Princeton, NJ 08543, USA
3)
University of Hyogo, Himeji, Hyogo 671-2280, Japan
4)
Institute for Advanced Fusion and Physics Education, Kita-ku, Kumamoto 861-5525, Japan
5)
University of Tokyo, Kashiwa, Chiba 277-8561, Japan
6)
Kyoto University, Kyoto 606-8502, Japan(Received 2 March 2017 / Accepted 24 March 2017)
Coaxial Helicity Injection (CHI) has now been implemented in QUEST. The goals for the first transient
CHI experiments were to establish reliable gas breakdown conditions, and to measure CHI-produced toroidal
current generation. Both these objectives were successfully met. Toroidal currents up to 29 kA were measured.
Interestingly, these first plasmas on QUEST also suggest the formation of small amounts of closed magnetic flux
surfaces.
c 2017 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: non-inductive current drive, CHI, spherical tokamak, solenoid free fusion plant, ECH
DOI: 10.1585/pfr.12.1202020
Establishment of methods for solenoid-free plasma
current start-up, with robust and stable non-inductive current drive on tokamaks increases the prospects for a compact and low aspect ratio fusion reactor, which has theadvantages of lower construction cost and higher plasmabeta. QUEST is a mid-size spherical tokamak (ST) device [1], in which plasmas are produced mainly by electron
cyclotron heating (ECH), and with minimal use of the central solenoid. Non-inductive toroidal plasma currents up to70 kA have been achieved on QUEST using 28 GHz microwave injection [2]. We have now implemented coaxial
helicity injection (CHI) capability on QUEST. We aim togenerate more than 100 kA of initial toroidal plasma current with transient CHI and ramp this seed current noninductively using the high-power 28 GHz microwave system. In this paper we report results from the first CHI experiments on QUEST.
CHI [3], a useful method for non-inductive current
drive, has been studied in the HIT-II and NSTX STs, inwhich the compatibility of CHI startup with conventional
Ohmic heating and current drive has been verified [4, 5]. Itis projected that the capability of CHI for current startup
would be significantly enhanced if ECH is used to heat theCHI plasma, and this is planned to be tested in NSTX-U
for a full non-inductive current start-up and ramp-up scenario [6].
author’s e-mail: kuroda@triam.kyushu-u.ac.jp

QUEST is equipped with important capabilities thatwill extend CHI studies to new parameter regimes. The internal vessel walls on QUEST are all metallic; this should
be beneficial to CHI as it would reduce the influx of lowZ impurities in to the plasma, which are the prime sourceof energy loss in low temperature plasmas. The CHI electrode configuration on QUEST is also simpler, and diﬀerent than the ones on HIT-II and NSTX, and it may be easier
to implement this configuration in a fusion reactor [7]. The
primary diﬀerence is that on NSTX and HIT-II, the insulator is also the vacuum boundary, whereas on QUEST, theinsulator is sandwiched between the electrode plate and thedivertor plate. However, CHI start-up in this new electrodeconfiguration needs to be demonstrated. This is an important part of the CHI program on QUEST.
Figure 1 shows the side view of the QUEST internal
structure. The nominal toroidal field at the machine axis
(at R0 = 0.64 m) is 0.25 T at its maximum limit. There
are four poloidal field coils above and below the vessel
mid-plane; these are normally operated in a series configuration, with two coils connected to a single power supply. As shown in Fig. 1, we installed an electrically insulated bias-electrode on the lower end-plate, and a corresponding ground-electrode is attached to the hot wall.
These toroidally continuous structures are the CHI electrodes. The ground-electrode has provisions for two CHIgas injectors at two toroidally displaced locations.

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c 2017 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Rapid Communications

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Fig. 1 QUEST internal structure and picture showing hot wall,
end-plates and CHI electrodes. Gray contours indicate
typical magnetic flux surface formed by poloidal fieldcoils, PF1-PF7. Orange and green lines show a fundamental harmonic ECR layer of 8.2 GHz and a second harmonic ECR layer of 28 GHz microwaves, respectively.

A CHI discharge sequence on QUEST involves thefollowing steps: 1) Toroidal field at Bt0 = 0.25 T is applied
and then the injector flux configuration is formed by driving current in four pairs of poloidal field coils. The injector
flux connects the bias and ground electrodes as shown inFig. 1. 2) The CHI gas valves inject H2 gas in the injector
region between the two electrodes. 3) At the time when gas
pressure in the injector region reaches a high value (about
10 ms after the valves are opened), about −1.8 kV of voltage is applied to the bias-electrode using a capacitor bankpower supply. This causes the injected gas to ionize andthe resulting injector current along the injector flux generates the CHI plasma.
In the first CHI experiments conducted on QUEST weinvestigated the conditions needed to attain gas breakdown.
For attaining reliable breakdown we had to use 20 to 30 kWof 8.2 GHz ECH for pre-ionization before step 2. For themagnetic configuration shown in Fig. 1, it was found thatbreakdown could be reliably obtained when the current inPF5-2 coil below the electrode, which is the main CHIinjector coil, was below 40% of its maximum current allowance rating. The breakdown became unstable when thePF5-2 coil current was higher than this value. This wasattributed to the need for an adequate magnetic field line
connection length between the cathode bias-electrode andthe anode ground-electrode to satisfy the Paschen condition for breakdown. Future experiments will expand thebreakdown conditions to higher levels of injector flux by
improving the gas injection system to allow it to transiently
increase the gas pressure in the injector region while simultaneously reducing the amount of injected gas.
Figure 2 shows that the voltage across the electrodes
rapidly drops at the time of breakdown. This is the firstindicator of the electrode being electrically connected tothe vessel through conductive plasma. The first millisec-Fig. 2 Shown are the measurements from fast voltage monitors across the electrodes, the CHI injector current andtoroidal current.

ond shows the injector current to increase, and with it thetoroidal current also increases and reaches a peak value of
29 kA. The toroidal current continues to increase as the injector current decays in time. In some of these discharges,
the toroidal current is present even after the injector current has been reduced to zero. These are indications of thepresence of a closed flux plasma decaying in time, after
the externally driven injector current, which flows on open
field lines, is reduced to zero. This observation, combinedwith other observations from a slow mid-plane camera (notshown here), which shows a bright region moving from thelower end-plate to the upper end-plate, indicates that theCHI plasma must be disconnecting from the injector fluxin the lower end-plate and drifting up. Such plasmas mustcarry closed field line currents, as they are no longer connected to the injector flux [8]. In these first experiments,
because the PF3-2 and PF5-2 coils are driven in pairs, thePF3-2 coil above the upper end-plate is in a polarity so
as to attract the CHI plasma. In future experiments, theupper coil will be driven with the current opposite to theCHI-produced toroidal current so as to trap the CHI plasma
at the vessel mid-plane and to provide plasma equilibrium
control. Images from a downward looking fast camera on
the upper part of the vessel (not shown here) show the visible emission to be most spread out at the time of peaktoroidal current. Better resolution is needed from the midplane camera to correlate it with the observations from theupper camera; this will be studied in future experiments.
In summary, these first results obtained from thenewly installed transient CHI system on QUEST are very
encouraging for future CHI studies on QUEST. Reliable
gas breakdown, an essential first step, has been successfully demonstrated in a new CHI electrode configuration.
The generation of toroidal current has also been successfully measured. These first plasmas carry up to 29 kA oftoroidal current. The data also suggests that some of thiscurrent must be flowing on closed field lines. Future ex-1202020-2

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periments will aim to increase the toroidal current and tocontrol the equilibrium of the CHI plasma, followed byinitial test of heating the CHI plasma with 28 GHz ECH.
This work is supported by US DOE grants DE-FG0299ER54519 and DE-AC02-09CH11466, NIFS bilateralcollaboration project NIFS14KUTR103 and RIAM international collaboration frame work No.9.

[2] H. Idei et al., Proc. 26th IAEA Fusion Energy Conference(2016) EX/P4-50.
[3] T.R. Jarboe, Fusion Sci. Technol. 15, 7 (1989).
[4] R. Raman et al., Phys. Plasmas 11, 2565 (2004).
[5] R. Raman et al., Phys. Plasmas 18, 092504 (2011).
[6] R. Raman et al., Nucl. Fusion 53, 073017 (2013).
[7] R. Raman et al., Fusion Sci. Technol. 68, 674 (2015).
[8] F. Ebrahimi and R. Raman, Phys. Rev. Lett. 114, 205003(2015).

[1] K. Hanada et al., Plasma. Fusion. Res. 5, S1007 (2010).

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